Molecular Motion in Liquids and Solutions. II. Dynamic Nuclear Polarization and Nuclear Magnetic Relaxation in Free Radical Solutions of Toluene

Two methods are utilized to study the relaxation behaviour and molecular motion in solutions of free radicals in toluene: the investigation of longitudinal and transversal solvent proton relaxation at a properly chosen frequency (48 MHz), and the determination of enhancement factors of the dynamic proton polarization over a large frequency range. The temperature dependence of the quantities concerned is measured between + 7 0 °C and —120 °C. The results can be interpreted in terms of a pure dipole-dipole interaction between proton and electron spins whose time dependence is provided by translational random motions of the molecules and rotational tumbling of associated complexes. The activation energies for both processes are 3.1 kcal/mole and 7.8 kcal/mole, respectively. The solvation is only very weak. The relative contribution of rotational tumbling is R— 0.26, from which the average co-ordination number is estimated to be less than one.